The invention relates to a circuit for converting an input d.c. voltage into an output d.c. voltage which is substantially independent on variations of the input voltage and/or variations of a load connected to the output voltage and comprising a first diode which is connected to a first terminal of the input voltage and a second diode connected to the second terminal of the input voltage, the first and second diodes being arranged in series with the same conductivity direction, and a series network formed by an inductance and a capacitor being arranged in parallel with the first diode, the circuit further comprising a winding one side of which is coupled to the junction between the inductance and the capacitor and the other side to an electrode of an in operation periodically controlled controllable switch, the other electrode of the switch being connected to the second terminal of the input voltage while a further diode is coupled by means of one side to the junction between the first and second diodes and by means of its other side to the junction between the winding and the switch, this further diode having the same conductivity direction as the switch, the winding being part of a resonant network which also comprises a tuning capacitor, a rectifier being coupled to the capacitor for generating the output voltage, in operation a saw-tooth-shaped current flowing through the winding while energy is stored in the inductance for replenishing losses and the conductivity period of the switch depending on the output voltage or a voltage proportional thereto.
Such a circuit is disclosed in the publication "Philips, Electronic components and materials: Technical note 082", published in 1978. One or more stabilized supply voltages are obtained with this prior art circuit. It has been found that the voltage across the capacitor is proportional to the input voltage and to the ratio of the conductivity period of the controllable switch and the period of the drive signal thereof, so that this voltage and consequently the output voltage can be kept substantially constant by controlling the said ratio. Tuning by means of the tuning capacitor assures that the voltage across the switch does not become too high during the cut-off period and that ringing which might be caused by parasitic capacitances and might produce an unwanted radiation is prevented from occurring.
For the adequate operation of the circuit said ratio must remain between predetermined limit values, so that the voltage across the capacitor and consequently the output voltage may not have any arbitrary value. In addition, as the tuning frequency of the resonant network cannot be of any low value for a predetermined switching frequency, for example because the present supply voltage is combined in a television receiver with the line deflection circuit thereof the situation may occur where the voltage across the switch during the cut-off period thereof exceeds the maximum value prescribed for this element.